The present invention relates, in general, to active antenna arrays, and more particularly to an improved beam steering technique for such arrays.
In radio frequency communication systems such as radar, wireless telecommunication, or the like, it is desirable to direct electromagnetic radiation from a transmitter to a target with the greatest efficiency possible, not only to reduce the required power, thereby reducing expense, but also to prevent interference. In radar applications, it is known to focus radiation from an array of antennas into a beam to illuminate an object to thereby generate a reflected signal. A high power source is required for this, with motor drives or multiple phase shifters being required to scan the antennas and the beam through a predetermined angle.
In wireless telecommunications, such as mobile telephones, arrays of antennas such as those used with radar have not been practical, with the result that omni directional antennas, which have a transmission pattern of about 360.degree. around the antenna, are generally used. Although such antennas do not require beam scanning, and thus are able to communicate with a plurality of receivers or with a single receiver having a location which changes as the mobile transmitter moves with respect to it, they experience significant difficulties. Not only does the need to transmit in all directions reduce the effective power available to be received by a particular receiver, but with multiple receivers, cell to cell interference can present a significant problem. Furthermore, an antenna which radiates in all directions may present a hazard to the user of the equipment who is exposed to the electromagnetic radiation.
In order to reduce cell-to-cell interference, to improve safety, and to reduce the amount of power required to drive the antenna, a directional antenna focusing the transmitted beam to the selected receiver would be required. Directional antennas are known, and typically have a 60.degree. wide transmission pattern, but such antennas are not practical in a moving vehicle, for example, which is moving with respect to the receiver location. A rotatable directional antenna would be required in such a situation to track the receiver, but such antennas present both mechanical and electrical problems.
The need to mechanically rotate an antenna, for example, introduces the requirement for a drive motor and its controls. These not only increase the cost of the unit, but are highly undesirable in, for example, an automobile or other small vehicle from which such transmissions are to be made. Furthermore, a rotary antenna present electrical problems in that it is difficult to maintain a reliable electrical connection between the stationary and the rotating components. Because of these practical difficulties, the users of mobile telecommunications equipment have had to continue the use of omnidirectional antennas, and to accept the consequent problems presented by cell-to-cell interference, and have had to accept relatively low power levels in order to prevent possible injury to users of the equipment, thereby reducing the effective range of such equipment. However, the need for a steerable, directional antenna without mechanical rotators to reduce the power required for such equipment, to reduce the exposure of nearby people to radiation, and to overcome cell-to-cell interference still exists.
An active quasi-optical array is an ensemble of antennas and active devices which are integrated into a planar substrate. Such arrays are well known with each active device having its own antenna that radiates energy into free space. Although most of the research on active arrays has concentrated on obtaining a fixed beam, these quasi-optical arrays are similar to antenna arrays in that the phase difference between active devices, or between antennas, determines the direction of the main radiating beam. Various techniques based on optics or electronics have demonstrated some control over the phase difference between oscillators in such active arrays. Thus, one way to steer a beam in an active array is to apply a signal with a differential phase difference between the ends of the array. (See K. D. Stephan and W. A. Morgan, "Analysis of Inter-Injection-Locked Oscillators for Integrated Phased Arrays", IEEE Transactions on Antennas and Propagation, Vol. AP-35, pp. 771-1084, July 1987.) In such an array, if all of the interior oscillators have the same frequency, then the phase difference applied between the ends of the array evenly distributes over the array. Thus, if there are n interior oscillators in a linear array, and if a signal having a phase difference of .phi. is applied to the ends of the array, the phase difference between adjacent oscillators is .phi./(n+1). In such an array, the maximum phase difference between the ends is limited to .+-.180.degree., so as the number of oscillators increases the maximum phase difference between adjacent oscillators decreases to 0.
Another technique is similar to the foregoing but offers an improved adjustable phase difference by injecting a frequency difference (rather than a phase difference) at the ends of the array. (See, for example, P. Liau and R. A. York, "A New Phase-Shifterless Beam-Scanning Technique Using Arrays of Coupled Oscillators", IEEE Transactions on Microwave Theory and Techniques, Special Issue on Quasi-Optical Techniques, October 1993.) In such a technique, the frequencies of the two oscillators at the ends of the array are set differently than the interior oscillators. However, the end oscillators will lock in at the same frequency as the interior oscillators, and the entire chain of oscillators will operate at the same frequency. The end oscillator which is set at a higher frequency tends to pull the chain along toward the higher frequency, whereas the end oscillator set at a lower frequency tends to drag the chain down in frequency. This pulling and dragging causes a phase lead and lag, respectively, between adjacent oscillators along the array which can continue until the end oscillators break away from the interior oscillators and start operating at different frequencies.
Although both techniques have benefits because of their simplicity, the synchronization of the set frequency, sometimes less than 1 MHz out of 10 GHz, of an oscillator array is difficult. In addition, these techniques both assume uniform energy exchange between neighboring oscillators, but to assure this, is necessary to provide additional circuitry which complicates the array.